Many image forming apparatuses such as photocopiers or laser printers adopt an electrophotography mode in which the surface of a photo conductor serving as an image carrier is charged by a charging device, and then is exposed by an exposure device. The resulting electrostatic latent image on the photoconductor is thereafter developed by a development device to be a toner image. Further, the toner image is electrostatically transferred to a transfer medium such as a recording paper by a transfer device, and the toner image transferred to the recording paper is finally fixed to the paper by a fixing device. An image is thus formed on a recording paper.
A general charging device is broken into a noncontact charging mode and a contact charging mode. The noncontact charging mode generally uses so-called a corotron charging device or a scorotron charging device. These charging devices generate corona discharge, and supplies electric charges to the photo conductor through the air. In such a noncontact charging mode, the charging device does not come in contact with the photo conductor, and therefore it causes less contamination or ablation of the photo conductor, which is a certain advantage. On the other hand, the corona discharge generates by-products such as ozone, which is not desirable.
In recent years, with the increased ecology consideration, a charging device of a contact charging mode not using corona discharge is attracting attention. A charging device of a contact charging mode includes a rubber roller member, which is charged by a predetermined voltage and is brought into contact with a photo conductor. The roller containing a rubber member is generally called a charging roller.
However, in the charging device of a contact charging mode, the photo conductor and the charging member are directly in contact with each other, and residue toner on the photo conductor or contaminant such as paper dust is adhered to the charging member, which causes inadequate charging.
In view of this problem, Patent Document 1 (Japanese Unexamined Patent Publication Tokukaihei 6-175466 (published on Jun. 24, 1994), corresponding US patent: U.S. Pat. No. 5,418,605) teaches a technology of preventing adhesion of contaminant to the surface of the charging roller by increasing a contact angle of water (increasing the hydrophobic property) on the surface of the charging roller (conductive roll) to be greater than the contact angle of water on the surface of the photo conductor. The contact angle of water designates an angle created by the surface of a water droplet and the surface of a solid at a contact point therebetween when the solid and a free surface (interface with the gas phase) of the water droplet are in contact in an equilibrium state. This angle is illustrated in FIG. 10.
However, in the technique of Patent Document 1, an increase in contact angle of water on the surface of the charging roller weakens the grip force between the charging roller and the photo conductor.
Therefore, in a structure where the charging roller is rotated due to the driving force of the photo conductor which is transmitted from the photo conductor as the charging roller comes in contact with the surface of the photo conductor, charging roller slips, causing an image defect which appears as black stripes.
The slip of charging roller more frequently occurs in a high-speed process, or in a structure using a photo conductor coated with a lubricant agent. Further, in the case where only a direct current is applied to the charging roller, the image defect of black stripes due to the slip of charging roller appears more significantly.